It is known to provide an electrically driven vehicle whereby a battery provides electrical power through an inverter to a drive motor. The drive motor is generally connected to two wheels of the vehicle through a torque transfer assembly that has a transmission coupled to a differential gearbox via a drive shaft. The differential gearbox delivers drive torque to side shafts of the assembly that in turn deliver torque to each of two wheels of the vehicle.
It is a characteristic of torque transfer devices such as geared transmissions and differential gearboxes that backlash is present within each gear mesh thereof and also in supporting bearings. When the direction of torque transmission through the device reverses the backlash is crossed and this can result in the generation of an audible noise and/or jerk experienced by the driver or a passenger. It is to be understood that backlash may be caused by a combination of free play within gearboxes and the torsional compliance of driveline components. An example of backlash is when a vehicle is in an engine overrun operating condition, where a driver may for example have released both an accelerator pedal and a brake pedal of the vehicle, such that the vehicle is slowly decelerating, to a state in which the accelerator pedal is depressed and the vehicle is accelerating. As the powertrain transitions from a state in which energy is being put into the engine in order to overcome frictional forces and a state in which the engine is delivering energy to the powertrain, the driveline transitions from a state of negative torque to positive torque and NVH in the form of a heavy ‘thump’ may be heard and felt by occupants of the vehicle.
In some cases a hybrid vehicle may be capable of providing drive torque to front wheels of a vehicle from an internal combustion engine and an electrical drive to rear wheels of the same vehicle from an electric machine (and optionally to drive the electric machine via the rear wheels to effect regenerative braking). The hybrid vehicle may be arranged such that the front and rear wheels are not mechanically coupled, i.e. there is no prop shaft connecting the respective sets of wheels.
The proportion of the total required drive torque that is provided by each torque source may be controlled in order to improve the overall efficiency of operation of the vehicle.
It is recognised that the torque distribution between torque sources also affects stability of the vehicle. For example, steering feel and handling balance as experienced by the driver may be controlled by adjusting the relative torque distribution between the two sources. For example, if the electric drive system connected to the rear wheels is operating in an electrical generation mode, recharging a battery of the vehicle, and the road surface is relatively slippery, the amount of regeneration torque developed by the wheels may be limited by a controller of the powertrain in order to maintain stability of the rear wheels.
It is an aim of embodiments of the present invention to reduce noise and/or jerk caused by torque reversal in one or more components of a vehicle torque transfer assembly.